Monarch butterflies

There is a touch of irony that, in a country famous for its unique wildlife, New Zealand’s most identifiable butterfly is the monarch (Danaus plexippus). The monarch is considered a native because it became established here on its own, but it is originally from North America.

New Zealand’s most recognised butterfly

The monarch is recognisable for two reasons – its main habitat is the suburban garden and it’s our largest common butterfly. We welcome monarchs into our gardens by planting their larval food – milkweed species such as swan plants – and enjoy watching their amazing journey through metamorphosis. Monarchs are more sensitive to cold than our endemic butterflies. They do not survive the harsh winters, so are less common in the far south.

The monarch’s life cycle

The monarch butterfly has one of the best-known life cycles in the insect world.

During mid-summer, the process goes from egg to adult in a month or so. The butterfly has a lifespan of 60–70 days during the summer, but this extends to 6–7 months if the butterfly pupates in autumn.

Monarchs (and all butterflies) are sensitive to air temperature. Their growth is the strongest in warm weather, and cooler temperatures signal a change in behaviour. Monarch butterflies in North America migrate from the colder northern regions to overwinter in California and Mexico, with some of the butterflies travelling thousands of kilometres! As the temperature warms, they return north to reproduce.

Adaptations to life in New Zealand

Monarchs in New Zealand do not follow the same migration pattern as their northern relatives. They have adapted their migration behaviour to suit local conditions. When the air temperature drops to 12.8°C, monarchs flock together in overwintering sites. These sites tend to be in milder coastal locations where the temperature remains at least 10°C. Overwintering monarchs prefer sites that are sheltered from the wind, have trees with a rough bark surface on which to cling and have a nearby source of nectar. The butterflies are mostly inactive, but on warm days, they fly, bask in the sunlight and feed. When the temperatures warm up, butterflies move inland to reproduce.

Swarms form regularly to overwinter at places such as Tauranga Bay in Northland. The Monarch Butterfly New Zealand Trust (MBNZT) was initially formed to protect this site. Other overwintering sites include areas of Hawke’s Bay, Nelson and Christchurch. The MBNZT encourages citizen scientists to tag butterflies so we can learn more about the migration and overwintering habits of monarchs.

Threats to the monarch

Monarchs use warning colouration and toxicity from their larval swan plant food source as defence mechanisms. This is enough to keep most vertebrate predators away, with the exception of the shining cuckoo.

Invertebrates are a different story. The brown soldier bug (Cermatulus nasalis) uses its hollow beak as a straw to suck the insides out of larvae. Praying mantids (Miomantis caffra Saussure and Orthodera novaezealandiae Colenso) eat them. The Tasmanian paper wasp (Polistes tasmaniensis) picks up larvae and carries them away.

An often-overlooked threat to monarch larvae is their dependence on swan plants. Without these plants, monarchs could not breed here. Four species of milkweed have been brought into New Zealand. The plants can reproduce naturally, but more likely, gardeners plant them to attract butterflies. The voracious larvae can quickly strip the plants, leaving hungry caterpillars to starve.

There are some other food sources available. Tweedia (Oxypetalum caeruleum) is a common garden plant with small blue flowers. Larvae will only eat the new shoots. A weed named the moth plant (Araujia sericifera) is another option. (Moth plant is an invasive weed, so in many regions of New Zealand, you’re not allowed to plant it but you can move your larvae onto it.) Mature larvae can eat pumpkin, but don’t try it with small caterpillars – they won’t survive. It’s worth remembering that monarchs lay a huge number of eggs, so losing a few larvae to predators or starvation is all part of the process.